The second of the Abrahamic religions, Christianity arose from the teachings of Yeshua of Nazereth, a Jewish carpenter commonly called by his Greek name Jesus, whom Christians believe to be the literal Son of God. Though Jesus was crucified in approximately 30 AD on the orders of Pontius Pilate, the Roman governor of Judea, at the behest of the Jewish High Priesthood, the faith he founded would ultimately supplant Hellenic paganism as the state religion of the Roman Empire following its legalization by Emperor Constantine I in 313. By the time of Crusader Kings II, Christianity is the dominant religion in Western and Central Europe, as well as in the Byzantine Empire and East Africa, and its reach continues to expand in later start dates by the conversion of West Asian pagans and the Spanish Reconquista.

There are four groups of Christians present in CKII: Roman Catholicism, Eastern Orthodoxy, Miaphysitism (also known as Copticism), and Nestorianism.

Sanctioned Invasion CB, grantable against any stronger ruler or any ruler whose title you have a claim on, should the Pope not like him (1000 piety)

The concept of a character's religious head is especially important for Christians as many characters may not have the same religious head even though they follow identical religions, due to (potential) antipopes and autocephaly.

Catholicism is the major religion of Western Europe. After each start date, it has a tendency to progress further east, and is almost always one of the most successful religions in the game.

Catholicism is one of the major instigators of the Great Schism. The Great Schism was a separation of the original Nicene Church after a prolonged rivalry between the Roman Pope and the Constantinopolan Ecumenical Patriarch, culminating in them excommunicating each other shortly before the "Stamford Bridge" game start (in 1054).

The religious head for Catholicism is the Pope, but it is not uncommon for one or more Antipopes to form throughout the game.

Bishops who like the Pope (or Antipope) more than their secular liege will shirk their duties as vassals. They will pay taxes to the Pope (or Antipope) and provide levies to nobody.

The only Christian denomination where crusades can be called against heretics and infidels. The Fraticelli heresy also features crusades if its religious head is created.

Catholicism and its heresies become heresies of Orthodoxy when an Orthodox character mends the Great Schism. If this happens crusades are disabled, even if Catholicism (or Fraticellism) becomes the mainstream.

Once the Crusades unlock, Catholicism gains more holy orders than any other religion in the game, up to five if their formation conditions are met. With Monks and Mystics, Catholicism gains two monastic orders, the Benedictines and the Dominicans.

Catholicism's holy sites are:

Rome: The seat of the Pope and historical capital of the Western Roman Empire.

Santiago: The site of the tomb of St. James the Great, one of the Twelve Apostles of Christ.

Köln: The site of the Shrine of the Three Kings, a reliquary said to contain the bones of the three wise men from the East who visited the young Christ in the Gospels.

Jerusalem: Holy to all Abrahamic faiths, Jerusalem is the historical seat of the Jewish faith and the site where Christ was executed and rose from the dead.

Kent: See of St. Thomas Becket, the Archbishop of Canterbury who was murdered by followers of King Henry II Plantagenet in 1170 AD (emblematic of the long-running struggle between secular and clerical rulers in Catholicism).

Fraticellism has identical mechanics to Catholicism, but the Fraticelli Pope (if created by a Fraticelli ruler who controls the county of Rome) is a duke-rank title. This means he may be vassalized by a mere king instead of only by an emperor as is the case with the Catholic Pope.

Catharism does not have a religious head or crusades, but allows female bishops and absolute cognatic succession law and has no penalties for female rulers or heirs. Bishoprics are filled by absolute cognatic open elective.

Lollardy and Waldensianism lack a religious head and crusades. Bishoprics are filled by agnatic open elective.

Eastern Orthodoxy is the religion of the Byzantine Empire and several of its neighbors. In 1066, it is also the religion of the Rurikid rulers (Rus-Ruthenia region).

The Byzantine Empire is by far the most powerful Orthodox ruler in most start dates. However, its rule is often challenged by heresy (notably Iconoclasty in 769) and powerful Muslim rulers.

The religious head of Eastern Orthodoxy is the Ecumenical Patriarch, usually residing in Constantinople. However, he only has jurisdiction over characters and provinces not already under pentarchs and autocephalous patriarchs. Most of the time, the Ecumenical Patriarch is also the Pentarch of Constantinople, and a vassal of the Byzantine Empire.

The only religion with a dangerous heresy at any start date (Iconoclasm in 769)

Mend the Great Schism to re-unite the Catholic and Orthodox religions if an Orthodox ruler owns Rome, Constantinople, Antioch, Jerusalem, and Alexandria as well as having the Ecumenical Patriarch as a vassal

If the Byzantine Empire does not exist, it can only be created by Orthodox rulers

The commander of the Varangian Guard will only convert to Orthodoxy (and not an Orthodox heresy)

Iconoclasty is functionally identical to mainstream Orthodoxy, though an Iconoclast Byzantine Emperor cannot request that the commander of the Varangian Guard convert to it. In the 769 bookmark, the emperor follows this heresy and the Iconoclast Patriarch rules in place of the Ecumenical Patriarch. The Iconoclast Patriarchate does not exist in other starts.

Bogomilism has no religious head nor autocephaly. It permits female bishops and has no penalties for female rulers or heirs, though not absolute cognatic succession law.

Other heresies follow the same mechanics as mainstream Orthodoxy but their own always-existent patriarchs.

Miaphysite Christianity, sometimes known as Oriental Orthodoxy or Coptic Christianity, is a religion that arose in opposition of the Council of Chalcedon's decision of Christ being "of two natures". Once spreading across the Byzantine Empire's non-Greek land as well as the Kingdoms of Armenia, Nubia, and Abyssinia, its spread has been significantly hampered by the Islamic expansion and is in danger of being wiped out.

The religious head of Miaphysitism is the Coptic Pope. However, like the Ecumenical Patriarch, he only has jurisdiction over a relatively small amount of characters and land. More info here. The Coptic Pope is often also the Bishop of Alexandria, if it is controlled by Miaphysite lords.

Relative minority religion, with most characters of the religion being independent counts and dukes in Eastern Africa, as well as some rulers in or near Armenia

Cannot mend the Great Schism

Miaphysitism gains a holy order, the Order of Saint Anthony, if Alexandria, Makuria, and Axum are Miaphysite and owned by Miaphysite characters. With Monks and Mystics it also has a monastic order, the Community of Saint Anthony.

Miaphysitism's holy sites are:

Alexandria: Historical seat of the Coptic Papacy. Pope Dioscorus I of Alexandria, who was deposed by the Council of Chalcedon, is recognized as the first Patriarch of the Coptic Church.

Aksum: The Kingdom of Axum declared Christianity its state religion in 330 AD.

Antioch: Severus of Antioch was the last Monophysite Patriarch of Antioch and is venerated as a saint in Miaphysitism.

Jerusalem

Napata: See of the Monophysite Bishop Longinus, and capital of the Nubian Kingdom which began converting to Christianity in the 6th century AD.

Miaphysitism's heresy Monophysitism is mechanically identical to the mainstream religion.

Nestorianism is an extremely minority religion in isolated areas near Baghdad and in an island near Arabia, in early starts. Once, Nestorianism dominated the Eastern world. However, the rise of Islam nearly wiped the faith off the face of the planet. There are few Nestorian starts and they will most likely be wiped out by expansionist Muslims or Mongols; the Khagans of the Golden Horde and Ilkhanate follow this religion in 1255 and 1260 respectively.

The religious head of Nestorianism is the Patriarch of the East, and similar to Orthodoxy and Miaphysitism, he only has jurisdiction over a small amount of characters. More info here. In the unlikely event that Antioch gets controlled by Nestorian rulers, the Patriarch of the East will also control the Bishopric of Antioch.

Minority religion, with less than 6 provinces following the religion at any time.

Cannot mend the Great Schism

Nestorianism gains a holy order, the Order of Saint Addai, if Jerusalem and Baghdad are Nestorian and owned by Nestorian characters. With Monks and Mystics it has a monastic order, the Community of Saint Abraham.

Due to the typical lack of moral authority it is relatively easy and harmless to convert from Nestorianism to its heresy. Messalianism has the Divine Blood feature, permitting and encouraging marriages to one's own siblings, parents, or children. It lacks a religious head and autocephaly, and allows absolute cognatic succession and has no penalties for female rulers or heirs.

Nestorianism's holy sites are:

Alexandria: Patriarch Cyril of Alexandria was the most ardent opponent of Nestorius's theology and was influential in having him declared a heretic at the Council of Ephesus, resulting in the Nestorian Schism.

Antioch: Nestorius received his clerical training and developed his ideas in Antioch, and briefly retired there after his teachings were declared heretical before later being exiled.

Baghdad: Seat of the Nestorian Patriarch from 775 to ~1310.

Jerusalem

Rayy: First mentioned as a diocese in 410, Rayy was elevated to a metropolitan see in 790. Fell into relative obscurity with the Islamization of Persia, with last mention of a Nestorian metropolitan in 1075.